Originates from Blizzard's game Starcraft where zerg were one of 3 playable races, aliens like appeareance, zerg were characterized by using large numbers of weak units to swarm the enemy, also another charasteristic of zerg were that everything was organic, ie, zerg didnt used machines or any artifacts, they would mutate their own buildings from special kind of larvae and advanced units were created by mutating basic ones. Units were cheap to make and it was possible to make extreme numbers in short period of time.

Nowadays term zerg is used in mmo games to describe force consisted of large group of lower level players (often with only basic equipment) who use numbers rather then strategy to defeat the enemy, therefore requiring no skill. This tactics is commonly known as zerging

Zerg as a verb is often used in conjunction with rush. The original use of the wort zerg was actually a specific tactic in the game of Starcraft to provide an early checkmate to an otherwise very balanced game of real time strategy. It involved finding the enemy early, and sendign as many of the basic Zergling units into your enemies base. The enemy would usually be able to beat off the first few waves of the lightly powered units, but at a cost to production. Since the person playing the Zerg race would commit almost all of thier resources to building the zerglings rather than upgrading buildings and technology they would slowly be able to wear down the production of the enemy until they could no longer generate enough of an income to fund the war. It is a tricky gambit because by putting all of your resources toward war early if the enemy is able to somehow form a defense you will find yourself at a serious disadvantage mid game. If a player on the opposing team knows that a zerg rush is coming or strongly suspects it they will build a defensive structure in their base. When I played Starcraft we usually had a no-zerg rule because it made the games less fun. Either the zerg rusher won in the first few minutes, or the person who deflected the zerg rush would win shortly after. By imposing a ten minute non-expansion peace treaty some of the best epic battles were fought.

In MMORPG's a popular way to beat a battle is by using overwhelming numbers, and the term zerging was borrowed from starcraft to define this. This can often accomplish a feat with minimal skill and or gear, however many games now take this into effect and design encounters so that after a certain threshhold of players it becomes more difficult to manage the encounter. For example, a few well placed hidden pits around a boss that when someone falls into them it releases a wave of ravenous creatures against the raid. In this case having a hundred people almost completely ensures that some idiot will fall into the traps no matter how many times you tell everyone where they are and how not to fall into them.

In other MMORPG's which are player vs player zerging is often seen when one team or player is playing so far above the other teams that the losing side finally builds up a force large enough to win purely by flesh count. DAOC was a game where this occured often by placing barriers to PVP that forces could camp. If the force camping the bottleneck into the frontiers was good then it would do one of two things. 1. It could demoralize the enemy and shut down RVR. 2. It would create a buildup of players behind the bottleneck that would eventuall form a zerg.

Zergs were considered by some a necessary evil in DAOC, but many considered them a design flaw in the game.

Starcraft zerging.
Human Vs. Zerg.
Building SCV (harvester) units.
Two zerglings (basic zerg fighter) attack before your first marine (basic human fighter) is built. You may attack the zergings with your SCV units, and will likely kill them, but at a cost of the life of an SCV unit or two. After a brief time you may get a marine or two built, but then six zerglings attack, and instead of attacking the marines they attack only the SCV units. The zerglings are weak and die quickly but may take out another couple SCV units. After another brief time the humans are trying to create more SCV units to boost production to a much needed level instead of creating more defenses when the next wave of 12 zerglings come in and decimate the remaining SCV units. As expected within minutes the game is over when the humans are overrun with waves of zerglings who have never had a shortage of raw materials.

EQ zerging: We are going to zerg rush the Avatar of War. - 30 minutes later the entire zone floor is covered in corpses, and the Avatar of War is standing above a hunred corpses like nothing ever happened.

DAOC Zerging.
From the Hibernians: I can't believe our 8 person group was fighting honorably when those Albion's came over the hill with a hundred people and frickin zerged us.
From the Albions: Cry moore noob hibblets.